"Autotransporter" proteins are the largest family of secreted proteins among gram-negative bacteria. A large number of autotransporter proteins, including several that are present among Category B Priority Pathogens, are known to have significant roles in pathogenesis. A distinguishing feature of autotransporters is the ability to mediate the translocation across the outer membrane of an intramolecular passenger (alpha) domain that performs a specific activity in the extracellular space. Our data indicate that at least one autotransporter (Shigella IcsA) is folded in the periplasm and suggest that it may remain folded during outer membrane translocation. Published data indicate that the periplasmic chaperone DegP is required for efficient secretion of IcsA. In this R21 application, we propose to explore whether periplasmic chaperones are generally required during secretion of autotransporters. Our studies will focus on the DegP family of protease chaperones and the five autotransporters of the Category B Priority Pathogen Shigella. Aim 1. Exploration of whether DegP serves as a periplasmic chaperone of Shigella autotransporters generally; Aim 2. Exploration of whether the DegP homologs DegQ and DegS also function as periplasmic chaperones of Shigella autotransporters; and, Aim 3 Determination of whether the conserved C-terminal motif of autotransporters is recognized by DegP, DegQ, and DegS. These exploratory studies are highly likely to result in additional specific testable hypotheses relevant to disease caused by this Category B Priority Pathogen.